1. Field of the Invention
The present invention relates to an in-plane switching mode liquid crystal display device and a method for fabricating the same, and more particularly to an in-plane switching mode liquid crystal display device, which reduces loss in transmittance and improves reflectance, and a method for fabricating the same.
2. Discussion of the Related Art
A liquid crystal display (LCD) device, one type of flat display device that is receiving a great deal of public attention, changes optical anisotropy by applying electric fields to liquid crystal having both fluidity of liquid and optical characteristics of crystal. Compared to a related art cathode ray tube, the LCD device has a lower power consumption rate and a small volume and is produced into a large-size and high-definition unit, thus being widely used.
Such a liquid crystal device has a structure in which a color filter array substrate serving as an upper substrate and a thin film transistor (TFT) array substrate serving as a lower substrate face each other and liquid crystal having dielectric anisotropy is interposed between the two substrates. The liquid crystal display device is driven such that TFTs attached to several hundreds of thousands pixels are switched on and off through address lines for selecting the pixels and a voltage is applied to the corresponding pixels.
The liquid crystal display devices are driven in various modes according to characteristics of liquid crystal and structures of a pattern.
Specifically, there are a twisted nematic (TN) mode, a multi-domain mode, an optically compensated birefringence (OCB) mode, and an in-plane switching mode. In the TN mode, liquid crystal directors are twisted at an angle of 90°, and a voltage is applied to the liquid crystal directors so that the liquid crystal directors can be controlled. In the multi-domain mode, one pixel is divided into several domains, and main visual field angles of the domains have different directions, thereby implementing a wide visual field angle. In the OCB mode, a compensating film is attached to a substrate so as to compensate for the variation in phase of light according to travel directions of the light. In the in-plane switching mode, two electrodes are formed on one substrate such that liquid crystal directors are twisted on the parallel planes of orientation films.
The liquid crystal display devices are divided into transmissible liquid crystal display devices using a backlight as a light source, reflective liquid crystal display devices using external natural light as a light source, and semi-transmissible light crystal display devices proposed to overcome drawbacks of the transmissible and reflective liquid crystal display devices, such as a high power consumption rate of the transmissible liquid crystal display devices due to use of the backlight and a difficulty of using the reflective liquid crystal display devices when the external natural light has a poor brightness.
The above-mentioned semi-transmissible light crystal display device simultaneously has reflection portions and transmission portions in unit pixels, thus interchangeably serving as reflective and transmissible light crystal display devices as occasion demands.
Transmission portions of the transmissible and semi-transmissible light crystal display devices cause light emitted from the backlight through a lower substrate to be incident upon a liquid crystal layer to increase luminance, and reflection portions of the reflective and semi-transmissible light crystal display devices reflect the external natural light incident through an upper substrate, when the external natural light has a high brightness, to increase luminance.
Here, in order to respectively maximize the efficiency of the reflection and transmission portions, a dual cell gap structure, in which the cell gap of the transmission portions is approximately twice that of the reflection portions, has been proposed.
A method for applying a semi-transmission mode in-plane switching mode liquid crystal display device is proposed. In this case, electrodes of the liquid crystal display device are configured in the dual cell gap structure, thereby maximizing the efficiency of the semi-transmission mode.
Hereinafter, with reference to accompanying drawings, an in-plane switching mode liquid crystal display device employing the semi-transmission mode will be described.
FIG. 1 is a plan view of a related art in-plane switching mode liquid crystal display device, and FIG. 2 is a sectional view taken along line I-I′ of FIG. 1.
The in-plane switching mode liquid crystal display device having pixel regions, each divided into reflection portions (R) and a transmission portion (T), as shown in FIGS. 1 and 2, comprises a TFT array substrate 11 having a plurality of lines and TFTs, a color filter array substrate 21 facing the TFT array substrate 11, and a liquid crystal layer 31 interposed between the substrates 11 and 21. The liquid crystal display device employs a dual cell gap structure in which the cell gap of the liquid crystal layer 31 at the transmission portions (T) is twice that of the liquid crystal layer 31 at the reflection portion (R).
Specifically, the TFT array substrate 11 comprises gate lines 12 and data lines 15 orthogonally crossing each other to define pixel regions, TFTs obtained by laminating gate electrodes 12a, a gate insulating layer 13, a semiconductor layer 14 and source/drain electrodes 15a and 15b at the crossing of the two lines 12 and 15, reflection electrodes 60 formed at the reflection portions (R) for reflecting external light, a passivation layer 16 formed on the data lines 15 and the reflection electrodes 60, and common electrodes 24 and pixel electrodes 17 crossing each other on the passivation layer 16 for generating transversal electric fields.
While the gate insulating layer 13 and the passivation layer 16 at the reflection portions (R) remain, the gate insulating layer 13 and the passivation layer 16 at the transmission portions (T) are removed, thereby forming a dual cell gap structure. Since the total sum of the thicknesses of the removed gate insulating layer 13 and passivation layer 16 is equal to that of the liquid crystal layer 31, the liquid crystal cell gap at the transmission portions (T) is twice the liquid crystal cell gap at the reflection portions (R).
As mentioned above, the cell gap (d1) at the transmission portions (T) and the cell gap (d2) at the reflection portions (R) is in the ratio of approximately 2 to 1. Thereby, ON/OFF modes of the transmission portions (T) and the reflection portions (R) are matched with each other.
Specifically, light incident upon the reflection portions (R) and light incident upon the transmission portions (T) simultaneously reach the surface of a screen. Natural light incident from the outside upon the reflection portions (R) reciprocates in the liquid crystal layer 31 and reaches the surface of the screen, and light incident from a backlight upon the transmission portions (T) passes through the liquid crystal layer 31 at the transmission portions (T) having a cell gap twice that of the liquid crystal layer 31 at the reflection portions (R) and reaches the surface of the screen. Accordingly, the above two lights simultaneously reach the surface of the screen.
The reflection electrodes 60 are made of Al, Al alloy, or Ag, and reflect light incident from an external light source, thereby displaying an image on the screen.
In the above device having the dual cell gap structure at the reflection and transmission portions (R and T), the common electrodes 24 and the pixel electrodes 17 are disposed in parallel at both edges of the transmission portions (T) without the passivation layer 16 and the reflection portions (R) with the passivation layer 16, thereby respectively forming first transversal electric fields (E1) and second transversal electric fields (E2). Specifically, the first transversal electric fields (E1) are formed throughout the cell gap (d1) of the transmission portions (T) by the interaction between the first common electrode 24a and the first pixel electrode 17a and the interaction between the second common electrode 24b and the second pixel electrode 17b, and the second transversal electric fields (E2) are formed throughout the cell gap (d2) of the reflection portions (R) by the interaction between the first pixel electrode 17a and the second common electrode 24b and the interaction between the second pixel electrode 17b and the first common electrode 24a. 
When an external light source is not present, the liquid crystal display device is driven in a transmission mode by the first transversal electric fields (E1) formed at the transmission portions (T), and when an external light source is present, the liquid crystal display device is driven in a reflection mode by the second transversal electric fields (E2) formed at the reflection portions (R).
Widths of the transmission portions (T) and the reflection portions (R) are varied according to the size of the liquid crystal display device, which is substantially manufactured, or the number of pixels of the liquid crystal display device. In consideration of the transmittance of the liquid crystal display device, preferably, the widths of the transmission portions (T) and the reflection portions (R) are in the ratio of 1:1 to 3:1.
The color filter array substrate 21 comprises black matrices 22 for preventing light leakage, and a color filter film 23 formed between the black matrices 22.
For reference, although not shown in the drawings, the liquid crystal display device further comprises orientation films attached to inner surfaces of the two substrates 11 and 21 for arranging molecules of the liquid crystal layer 31 in a designated direction, polarizing films attached to outer surfaces of the two substrates 11 and 21 for controlling an optical axis of light, and a phase contrast plate interposed between the color filter array substrate 21 and the polarizing film for delaying a phase difference.
The above-mentioned related art in-plane switching mode liquid crystal display device has a problem, as follows.
When each of the reflection electrodes made of metal is disposed between the corresponding common electrodes and pixel electrodes, the reflection electrodes distort the transversal electric fields generated between the common electrode and the pixel electrode, thereby causing a difficulty in arranging liquid crystal molecules in a desired direction.